Fuel cells have been developed as alternative power sources for motor vehicles, such as electrical vehicles. A fuel cell is a demand-type power system in which the fuel cell operates in response to the load imposed across the fuel cell. Typically, a liquid, hydrogen-containing fuel, for example, gasoline, methanol, diesel, naphtha, etc. serves as a fuel supply for the fuel cell after the fuel has been converted into a gaseous stream containing hydrogen. Such liquid fuels are particularly desirable as the source of the hydrogen used by the fuel cell owing to their ease of on board storage and the existence of an infrastructure of service stations that can conveniently supply such liquids. The conversion to the gaseous stream is usually accomplished by passing the fuel through a fuel reformer to convert the liquid fuel to a hydrogen (H2) gas stream that usually contains other gases such as carbon monoxide (CO), carbon dioxide (CO2), methane, water vapor (H2O), oxygen, and unburned fuel. The hydrogen is then used by the fuel cell as the fuel in the generation of electricity for the vehicle.
A polymer electrolyte membrane (PEM) type of fuel cell is generally composed of a stack of unit cells comprising a polymer electrolyte membrane enclosed between electrodes and gas diffusion layers, and further enclosed between separators that contain channels for fuel gas and oxidant gas. The stack is fixed by end plates. A current collector may be provided between the end plate and stack, or the end plate itself may function as a current collector. When hydrogen is used as the fuel gas and oxygen is used as the oxidant gas, electrons are released due to a chemical reaction, and water is formed as a by-product, via the reaction:H2+½O2→H2O.Consequently, the fuel cell is an energy source that has no adverse impact on the global environment, and has been the focus of much research for use in automobiles in recent years.
PEM fuel cells include a “membrane electrode assembly” (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid polymer electrolytes are typically made from ion exchange resins such as perfluorinated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles, often supported on carbon particles, admixed with proton conductive resin.
Because the carbon monoxide produced in the fuel reformer acts as a poison to some fuel cells, such as PEM fuel cells, the carbon monoxide concentration in the hydrogen stream must be removed, or its concentration reduced for example by oxidation, conversion, or separation, before the hydrogen stream can be used in these fuel cells to produce electricity. Optional post-processing of the hydrogen stream to reduce the carbon monoxide content include selective catalytic oxidation and methanation.
For fuel cells such as PEM fuel cells which are sensitive to carbon monoxide, the hydrogen stream is passed to a carbon monoxide oxidation reactor at effective oxidation conditions and contacted with a selective oxidation catalyst to produce a hydrogen gas stream comprising less than about 40 ppm CO. Preferably, the hydrogen gas stream comprises less than about 10 ppm CO, and more preferably, the hydrogen gas stream comprises less than about 1 ppm CO.
The catalytic water gas shift conversion process is well known and is commonly used in processes which manufacture hydrogen gas to reduce CO. In the water gas shift reactor, carbon monoxide is combined with water to yield carbon dioxide and hydrogen according to the following formula:CO+H2O→CO2+H2.This reaction, commonly known as the water gas shift reaction, is highly exothermic, liberating about 16,700 BTUs for each pound mole of carbon monoxide converted. Water gas shift (WGS) reactors are often used to reduce the amount of carbon monoxide present in a gas stream typically composed of water vapor, methane, carbon monoxide, carbon dioxide and hydrogen.
Water gas shift reactors are particularly useful in hydrocarbon fueled electric power generation systems including PEM fuel cells. In these systems, fuel is first reformed in a fuel reformer to yield a mixture of hydrogen, carbon dioxide and small amounts of carbon monoxide. This gas mixture is commonly referred to as reformate. The reformate is produced by reacting fuel, air, and water vapor over a catalyst, such as nickel with amounts of other metal, such as cobalt, platinum, palladium, rhodium, ruthenium, iridium and a support such as magnesia, magnesium aluminate, alumina, silica, zirconia, singly or in combination. The reforming catalyst can be a single metal such as nickel or a noble metal supported on a refractory carrier such as magnesia, magnesium aluminate, alumina, silica, or zirconia, singly or in combination, promoted by an alkali metal such as potassium. The reforming catalyst can be granular and is supported as a fixed catalyst bed.
The reformate is then introduced into the WGS reactor, where the carbon monoxide concentration is reduced in order to avoid poisoning by the carbon monoxide of the catalyst employed in the fuel cells and to produce additional hydrogen fuel. Frequently, the reformate stream exiting the WGS reactor is introduced into a preferential oxidation (PROX) reactor, which further reduces the level of carbon monoxide present in the stream.
The conventional WGS reactor is an adiabatic bed in which the process gas temperature increases as the amount of carbon monoxide is reduced by the water gas shift reaction. Due to equilibrium limitations, catalyst activity, and catalyst thermal limits, conventional WGS reactors are generally incapable of reducing the carbon monoxide concentration of a reformate stream much below 2.0%.
The water-gas-shift reaction may be accomplished in a single low temperature shift reactor, or in a two stage shift reactor wherein the reformate stream first passes through a high temperature shift (HTS) reactor, and thence through a low temperature shift (LTS) reactor. A HTS reactor is typically an adiabatic shift reactor having a catalyst, such as iron oxide or chromium oxide, operable to effect the water-gas-shift reaction at about 300° C. to about 500° C. A LTS reactor is typically an adiabatic shift reactor having a catalyst, such as Cu—ZnO, operable to effect the water-gas-shift reaction at about 150° C. to about 280° C. A heat exchanger can be used to cool the reformate exiting the HTS reactor before it enters the LTS reactor.
The high temperature shift catalyst can be selected from the group consisting of iron oxide, chromium oxide and mixtures thereof. The low temperature shift catalyst can comprise cupric oxide (CuO) and zinc oxide (ZnO). Other types of low temperature shift catalysts include copper supported on other transition metal oxides such as zirconia, zinc supported on transition metal oxides or refractory supports such as silica or alumina, supported platinum, supported rhenium, supported palladium and supported rhodium.
Some CO survives the water-gas-shift reaction and needs to be reduced further (i.e. to below about 40 ppm) before the reformate can be supplied to the fuel cell. It is known to further reduce the CO content of H2-rich reformate exiting a WGS reactor by reacting it with oxygen (i.e. as air) in a preferential oxidation reaction carried out in a catalytic PROX reactor. The preferential oxidation reaction is exothermic and proceeds as follows:CO+½O2→CO2.The PROX reactor effluent, a CO-cleansed, H2-rich reformate, is then supplied to the fuel cell.
In a PROX reactor, a limited amount of air is selectively, exothermically reacted with the carbon monoxide, rather than the hydrogen, over a suitable catalyst that promotes such selectivity. When the system is operating under normal steady state conditions, the fuel stream exiting the PROX reactor is sufficiently CO-free so that it can be used in the fuel cell without poisoning the fuel cell catalyst.
Upon fuel cell shutdown, reducing gas (H2) is removed from the fuel processing system. This causes a loss of catalytic activity in the fuel reformer, WGS reactor, and PROX reactor, which normally operate in highly reducing atmospheres. Another problem introduced by fuel cell shutdown is that an inert circulating gas, such as N2, typically must be supplied to the fuel cell system. Because of the limited space in a motor vehicle a nitrogen storage tank is difficult to use in automotive applications. Furthermore, when circulating gas is introduced at system shutdown, the partly processed high-concentration CO in the system is released, unconverted, into the fuel cell and fuel cell exhaust. This causes CO poisoning of the fuel cell, impedes the subsequent restart, and causes emission of a regulated substance (CO) outside the system. In addition, since the CO is mixed with partly processed H2, flammable gases (CO and H2) are released in the fuel cell exhaust. Further, additional components, such as circulating blowers and circulation lines for the inert gas are necessary, which increase the space requirements and cost of the fuel cell system.